High temperature furnace



Dec. 3, 1968 T. B. REED 3,414,661

HIGH TEMPERATURE FURNACE Filed May l9, 1965 /NVENTOR THOMAS B. REED www ATTR/VE 3,414,66 l l Patented Dec. 3, 1 968 3,414,661 HIGH TEMPERATURE FURNACE Thomas B. Reed, Concord, Mass., assignor to Massachusetts Institute of Technology, Cambridge, Mass., a corporation of Massachusetts Filed May 19, 1965, Ser. No. 457,037 5 Claims. (Cl. 13-31) This invention relates to high temperature furnaces and'in particular to high temperature furnaces utilizing electric resistance heating elements.

Many new applications and substantial growth in existing uses of high temperature `furnaces in the eld of crystal growing, sintering, and various laboratory requirements have undergone strong development. One difficulty some furnaces must confront is the attainment of high temperatures while exposing the furnace to oxydizing and reducing atmospheres.

Gas furnaces will provide high temperatures in oxidizing atmospheres; but, the temperature control is not as line as electrical heating. Furthermore, electric heating is much cleaner, safer and overall m-ore desirable. One form of electric heating using radio frequency heating is available for operating Vin oxidizing atmosphere, but the heating elements must be preheated with another heating element until the unit becomes sufficiently conducting to permit the radio frequency supply to produce heating in the furnace. The cost of this equipment is excessive as well as presenting some hazards to operators.

Direct electric resistance heating provides ne control, is inexpensive, and superior in many other ways to other forms of heating. When the material being heated can be heated in a neutral atmosphere such as argon, many heating elements such as carbon or tungsten can be used to attain temperatures as high as 3000 C.

Many applications require that materials be heated in air or an oxydizing atmosphere. The highest temperature attainable with nichrome heating elements in such an atmosphere is 1200 C. Using a platinum heating element, temperatures as high as 1800o C. are attainable. With the present invention, temperatures of 2400 C. can be attained using tantalum heating elements.

Furthermore, the present invention can replace several special purpose furnaces as well as extend the high temperature` operation in oxidizing atmospheres to approximately 2400 C. The flexibility provided by this invention willenable users to meet nearly all their high temperature furnace needs with this one furnace. The temperature can be further extended to 3000 C. with slight modification. However, the need for temperatures above 2400 C. is presently seldom encountered.

Therefore, an object of this invention is to provide a very simple and inexpensive electric resistance-heated furnace,

Another object of this invention is to provide an electric furnace able to attain temperatures of approximately 2400 C. in an oxidizing atmosphere.

Another object of this invention is to provide a furnace able to attain temperatures of approximately 3000" C. in oxidizing atmospheres.

Another object of this invention is to provide a flexible electric furnace able to heat substances to high temperatures in an oxidizing atmosphere as well as a neutral or reducing atmosphere.

Other objects and features of this invention will become more apparent from the following description when taken with reference to the accompanying drawing which shows a cross sectional view of an embodiment of this invention.

High temperature furnace 13 is composed of two parts, an upper section 8 and a lower section 9. These sections are bolted together when the furnace is assembled. The

lower section is supported by means of posts 15 which are mounted on pedestals 12. Clamps 16 enable the furnace to be raised or lowered slightly on posts 15. Posts 15 contain power leads that extend from terminal blocks 11 down below the pedestal to a transformer not shown that supplies 8 kilowatts at 600 amps.

The lower section of furnace 13 is placed and mounted on posts 15 before heating elements 27 (by means of bars 21) is connected to terminal blocks 11.1Current from the transformer by way of the leads in posts 15 feeds terminal blocks 11 and thence through lead in bars 21 to tantalum heating element 27 which is split into two sections connected at the bottom as indicated by bar 22. Tantalum heat shields 37 are then placed vertical in the furnace to confine the heat to the center of the furnace. Lower and upper heat shields 25 are placed horizontal and maintain the base and top of the furnace c ool. A high density tube made from zirconia (such as supplied by the Zirconium Corporation of America) 51 is then placed free standing in the furnace. Heat shields 3 8 which are placed within zirconia tube y51 are made of ceramic and protect the tube from cracking when the furnace is operated at its highest temperatures. This will be explained more fully as we proceed, The top section of the furnace 8 is bolted to lower section 9 by means of bolts 7. Cooling water is circulated through tubing 14, which surrounds the entire furnace, when the furnace is in operation to maintain the outside of the furnace at nominal temperatures. It will be observed that heat shields 37 have holes that are lined up with site window 33. This enables the zirconia tube temperature to be determined by means of an optical pyrometer whenever necessary.

The ends of the zirconia tube 51 are not secured or fastened tightly, but are packed carefully with a refractory such as fibrous mullite about the top and bottom which serves to hold the tube in place and to seal the tube against the entry of unwanted gases. Likewise, lead in bars 21 of heating element 37 are also packed with brous mullite to seal the heating element chamber against the entry of air into the furnace. Lead-in conductors could be made a fixed part of the furnace wall, and thereby eliminate the need for packing with mullite.

It will be obvious that a furnace where the lead-in conductors are a xed part of the furnace wall the gas inlet and outlets can be closed so that the entire furnace evacuated to provide a vacuum furnace. Here, the zirconia tube 51 will merely serve to isolate the heating elements from the materials being heated.

Associated with site port 33 is gas inlet 32 wherein a gas, preferably argon, is pumped into the furnace during its operation. The surplus gas escapes out of the various minute openings around the furnace at the bottom and top of the tube and around the current leads 21 which pass to the outside atmosphere. Through inlet 31 another gas such as air or oxygen or even argon is introduced into the inside of zirconia tube 51 and is exhausted by way of the lower tube opening where shaft 34 enters the furnace.

Samples to be heated are placed on pedestal 36. Shaft 34 in conjunction with an electric motor can raise or lower the sample to be heated within the furnace through the various temperature zones.

When the furnace is operated at its highest temperature, the current coming from terminal 11 may cause the current lead-in bars 21 to burn out, consequently, they must be protected. Tantalum foil has been used effectively when wrapped around these terminals and oxidization of the copper near the furnace was eliminated.

Also associated with this furnace is a safety device 56 which is a filament of tungsten and is inserted in the furnace near the outermost heat shield. The lilament is protected from short circuit by insulation 57 which surrounds the filament wire as it passes through the furnace walls. If any air inadvertently enters the area surrounding the heating elements, the tungsten filament will burn out interrupting the current through a relay thereby shutting down the transformer supplying the furnace.

When the furnace is completely assembled, argon is rst flushed through the heating chamber element at about 15 liters a minute for about l0 minutes, after which the rate is reduced to about 5 liters a minute for operation. The desired gas (oxygen or air and so forth) is circulated through the zirconia tube at about 5 liters per minute, the power is raised gradually over a period of approximately half an hour until the operating temperature is reached.

Cracking occurs, if at all, in the zirconia tube within an inch from the upper and lower ends of the tube where the iibrofax seals are made. Thin ceramic shields 38 placed inside the tube at these locations will prevent cracking.

In a typical run, the temperature of the furnace is slowly raised for approximately one half hour by a gradual increase in supply current. The temperatures were measured by means of a calibrated laboratory optical pyrometer. The pedestal is observed through a right angle prism mounted in place of furnace lid 35. Sapphire was placed on pedestal 36 which is made of magnesium. The melting point of sapphire for example as observed through the prism was measured at 2050 C. which is in good agreement with published values.

In another run a temperature of 2400" was measured. However, the current lead must be protected in the area where they pass into the furnace from terminal blocks 11. They can be protected by means of tantalum foil which is wrapped around the leads to protect the current leads from oxidizing in room air.

The zirconia tube in the furnace is advantageous even in instances where the sample is not heated in oxidizing atmosphere since the zirconia protects heating elements 27 from the material that may evaporate from the sample.

In many cases in furnaces using tantalum heating elements, as in this case, operation in argon is much more satisfactory than in a vacuum. Argon decreases the rate of evaporation of both the tantalum heating element and the sample being heated. It also reduces the rate at which impurities are removed. However, commercial argon has a nominal maximum impurity level of about parts per million corresponding to an impurity partial pressure of 10-2 torr and is effectively gettered by hot tantalum. If the argon is introduced at port 32, the first heat shield will getter out the impurities as evidenced by discoloration of the outermost heat shield opposite the argon inlet. To avoid damaging any of the regularly used heat shield another shield may be added which will getter the incoming argon gas and/or the gas may be introduced at a more remote location in the heating element chamber.

Using high density zirconia as the inner tube surrounding the samples to be heated will assure that the tantalum heating element is protected from the evaporation of the heated sample and from oxidization. The high density zirconia tube is gas impermeable at elevated temperatures and prevents the flow of gases from the sample chamber to the heating element chamber. Placing the tantalum tube on fibrous mullite and otherwise free standing permits the tube to move freely as it is heated to the very high temperatures that this furnace is capable of attaining. The balancing of the gas pressure permits the tantalum heating clement to be free of oxygen or air entering the furnace inside of zirconia tube 51. If any oxygen inadvertently enters the tantalum heating element chamber, the tungsten filament will shut the furnace down.

Tantalum melts at approximately 2700o C. while tungsten melts at 3300 C. Furthermore, zirconium oxide melts at 2600 C. while thoria melts at 3200 C. Substituting, a tungsten heating element and tungsten heat shields for the tantalum heating element 27 and the tantalum heat shields 37 and 25, and a thoria ceramic tube (ThOz) 4 for the zirconia tube 51 (ZrO2) will permit the furnace to operate at higher temperatures up to approximately 3000 C.

In a furnace employing tungsten heating elements, the cooling will necessarily be increased and the number of heat shields should be doubled. The inner heat shields immediately adjacent heating element 27 would necessarily be tungsten but the outer shields could be tantalum.

The power supply to the above furnace must be four times as large as it would be with the tantalum heating element furnace, due to a decrease in radiation with reference to input current at such elevated temperatures.

While I have described the above principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is only made by way of example and not as a limitation on the scope of my invention as set forth in the objects thereof and in the accompanying claims.

What is claimed is:

1. A direct resistance heated furnace comprising, a furnace body, a tantalum heating element, a source of electric power, said source of electric power supplying heating current to said heating element, a source of cooling uid, said fluid caused to circulate Ithrough said furnace body maintaining said furnace at a preselected temperature, a zirconia tube dividing the interior of said furnace into two compartments, one containing said heating element and another surrounding a work area, librofax packing sealing the contact area of said zirconia tube 'and said furnace, said tantalum heating elements disposed in close proximity with said zirconia tube, tantalum heat shield, said heat shield disposed between said heating element and said furnace body thereby conning the heat within said furnace, ceramic heat shields disposed within said tube at each end, a source of argon gas coupled to said heating element compartment, a source of oxidizing gas coupled to said work area within said zirconia tube.

2. A direct electric resistance heated furnace described in claim 1 which further includes, a tungsten fuse, a relay, said fuse connected to said relay, and disposed within said heating compartment to open on exposure to oxygen thereby tripping said relay shutting down the power supplying said heating element.

3. A direct resistance heated furnace comprising, a furnace body, a tungsten heating element, a source of electric power, said source of electric power supplying heating current to said :heating element, :a source of cooling iiuid, said tluid caused to circulate through said furnace body maintaining said furnace at a preselected temperature, a thoria tube dividing the interior of said furnace into two compartments, one containing said heating element and another surrounding a work area, fibrofax packing ealing the contact area of said thoria tube and said furnace, said tungsten heating elements disposed in close proximity with said thoria tube, tungsten heat shields, said heat shield disposed between said heating element and said furnace body thereby confining the heat within said furnace, ceramic heat shields disposed within said tube at each end, a source of argon gas coupled to said heating element compartment, a source of oxidizing gas coupled to said work area within said thoria tube.

4. A direct electrical resistance heating furnace comprising a furnace body, a tantalum heating element, a source of electric power, said source of electric power supplying heating current to said heating element, a zirconia tube dividing the interior of said furnace into two compartments, one containing said heating element and .another surrounding work area, fibrofax packing sealing the contact area of said zirconia tube and said furnace, said tantalum heating element disposed in close proximity wit-h said zirconia tube, tantalum heat shield, said heat shield disposed between said heating element and said furnace body thereby confining the heat within said furnace, a source of neutral gas, a source of oxidizing gas, 'said source of oxidizing gas coupled to the compartment defining said work area, said neutral gas coupled to said chamber containing said heating element.

5. A direct electrical resistance heating furnace cornprising a furnace body, a tantalum heating element, a source of electric power, said source of electric power supplying heating current to said heating element, a zirconia tube dividing :the interior of said furnace into two compartments, one containing said heating element and another surrounding work area, brofax packing sealing the Contact Iarea of said zirconia tube and said furnace, said tantalum heating element disposed in close proximity with said zirconia ltube, tantalum heat shield, said heat shield disposed between said heating element and said furnace body thereby confining the heat within said furnace, a source of neutral gas, a source of reducing gas, said source of reducing gas coupled to the compartment defining said work area, said neutral gas coupled to said chamber containing said heating element.

References Cited UNITED STATES PATENTS 6/1925 Kelly et al. 219-406 2/1962 Casey 263-42 2/1965 Lewis 13-31 5/1965 Mescher et al 26340 6/1967 Clune et al. 13-20 5/ 1954 Kistler 13-20 l0/1956 Buck et al. 13-31 X 6/1963 Bukata 13-22 X 9/1963 Denton et al 13-31 X 3/1966 Sweet 266-24 FOREIGN PATENTS 7/ 1945 France.

BERNARD A. GILHEANY, Primary Examiner. 

1. A DIRECT RESISTANCE HEATED FURNACE COMPRISING, A FURNACE BODY, A TANTALUM HEATING ELEMENT, A SOURCE OF ELECTRIC POWER, SAID SOURCE OF ELECTRIC POWER SUPPLYING HEATING CURRENT TO SAID HEATING ELEMENT, A SOURCE OF COOLING FLUID, SAID FLUID CAUSED TO CIRCULATE THROUGH SAID FURNACE BODY MAINTAINING SAID FURNACE AT A PRESELECTED TEMPERATURE, A ZIRCONIA TUBE DIVIDING THE INTERIOR OF SAID FURNACE INTO TWO COMPARTMENTS, ONE CONTAINING SAID HEATING ELEMENT AND ANOTHER SURROUNDING A WORK AREA, FIBROFAX PACKING SEALING THE CONTACT AREA OF SAID ZIRCONIA TUBE AND SAID FURNACE, SAID TANTALUM HEATING ELEMENTS DISPOSED IN CLOSE PROXIMITY WITH SAID ZIRCONIA TUBE, TANTALUM HEAT SHIELD, SAID HEAT SHIELD DISPOSED BETWEEN SAID HEATING ELEMENT AND SAID FURNACE BODY THEREBY CONFINING THE HEAT WITHIN SAID FURNACE, CERAMIC HEAT SHIELDS DISPOSED WITHIN SAID TUBE AT EACH END, A SOURCE OF ARGON GAS COUPLED TO SAID HEATING ELEMENT COMPARTMENT, A SOURCE OF OXIDIZING GAS COUPLED TO SAID WORK AREA WITHIN SAID ZIRCONIA TUBE. 